Chemical vapor deposition (CVD) is a chemical process used to produce high-purity, high-performance solid materials. The process is often used in the semiconductor and photovoltaic industries to produce high quality silicon materials. In a conventional CVD process, a rod structure is exposed to one or more volatile precursors, which react and/or decompose on the rod surface to produce the desired deposit. Frequently, volatile byproducts are also produced, which can be removed by gas flow through a reaction chamber in the CVD reactor.
One method used to produce solid materials such as polysilicon by deposition in a chemical vapor deposition reactor is known as the Siemens method. When producing polysilicon using the Siemens method, polysilicon is deposited in a CVD reactor on one or more high-purity thin silicon rods, also known as “slim rods.” Typically, the slim rods must be heated to an elevated temperature to enable the deposition. According to the Siemens method, electrical current is passed through the slim rods to raise their temperature to approximately 1000° C., and in some cases to a temperature as high as 1200° C.
Because these slim rods are fabricated from high-purity silicon, the corresponding electrical resistance of the slim rods at room temperature is extremely high. Thus, a very high initial voltage is required to initiate the electric current during the startup phase of the CVD process. Typically, a high voltage on the order of thousands of volts is initially applied to the rods. Due to this high voltage, a small current can begin to flow through the slim rods. This current generates heat in the slim rods, reducing the electrical resistance of the rods, and permitting yet higher current flow which generates additional heat. As the rods heat up to the desired temperature, the applied voltage is correspondingly reduced.
A typical “Siemens” type polysilicon CVD reactor is illustrated in FIG. 1, reproduced from FIG. 1 of U.S. Pat. No. 6,284,312 which is incorporated in its entirety by reference herein. Generally, polysilicon rods are produced in a Siemens CVD reactor 100 by the pyrolytic decomposition of a gaseous silicon compound, such as mono-silane or a chlorosilane (e.g., trichlorosilane), onto the silicon slim rod, also referred to as the starter “filament.” The CVD reactor 100 includes a reaction chamber 24 defined by or having a baseplate 23 and an enclosure, often referred to as a “bell jar” 17 securable to the base plate 23. The bell jar 17 can be comprised of quartz, and/or of a metal such as any of the various grades of stainless steel alloys.
In the example shown in FIG. 1, the chamber contains an assembly of slim rod filaments in a hairpin configuration in the form of two vertical filaments 11, which are connected by a horizontal bridge 12. The filaments are heated by passing electrical current through them while being exposed to a silicon-containing gas, thereby causing silicon 13 to be deposited onto the filaments. Also, electrical feed throughs 19 and a gas inlet and outlet 20 and 21, respectively, can be incorporated into the base plate 23. A viewing port 22 can provide for visual inspection of the interior.
Connections between traditional slim rod filaments in a CVD reactor, and between the vertical filaments 11 and corresponding support chucks, are important to maintaining electrical connections in the reactor 100. With regard to the chuck-to-filament connections, known attachment mechanisms utilize screws, bolts, clamps, etc. Known connections between vertical filaments 11 and horizontal bridge 12 are formed with a groove or a key slot at the top of each vertical rod. A small counter bore or conforming figment can be formed on the ends of the horizontal bridge 12 so that it can be press fitted into the grooves to bridge the two vertical slim rods 11.
With reference to FIGS. 2A and 2B, and as described in U.S. Pat. No. 6,284,312, large diameter vertical silicon tube filaments have been utilized in place of slim rods. In FIGS. 2A and 2B, the vertical filaments 11 of FIG. 1 are replaced by vertical cylindrical tube filaments 200, which are joined by a flat silicon bridge 202 that rests on the tops of the vertical tube filaments 200. FIG. 2A illustrates the tubular filament assembly with the bridge 202 lifted off of the tubes 200 for clarity of illustration, while FIG. 2B illustrates the apparatus of FIG. 2A with the bridge 202 resting on the tops of the vertical tubes 200.
Tube filaments provide several advantages over traditional slim rod filaments. Due to the higher surface area of the tubular filaments 200, silicon is deposited at a faster rate. Also, under ideal conditions, an increased overall surface area of connection between the tube filaments provides decreased resistance at the connections, so that the total resistance that must be initially overcome is lower. Therefore, a lower voltage is necessary to induce current flow through a tube filament hairpin configuration, as shown in FIG. 2B, as compared to a conventional slim rod hairpin configuration, as shown in FIG. 1.
However, conditions are not always ideal. For example, the filament tops may not be cut exactly flat or perpendicular to their centerlines. In addition, the filaments may not be exactly the same height. Lab testing in a CVD reactor of hairpin filament assemblies formed by two vertical cylinders of silicon joined at the top by a flat silicon bridge has shown that maintaining adequate electrical connection between the components of the hairpin assembly is sometimes difficult, often as a result of these imperfections. The connection method works best if the tops of the tubes are in exactly the same plane. Any variance from this can cause the bridge to lose its flat face-to-face connection, so that the bridge rests only on point contacts on one or both tubes. As a result, electrical current will tend to flow only at the points of contact between the tube tops and the bridge, leading to higher resistance and possible local heating issues.
FIG. 2C is a side view of the hairpin filament assembly of FIG. 2A shown under ideal circumstances. FIG. 2D illustrates the same filament assembly, shown with the right-hand vertical cylinders tilted by 2°. Note how the flat bridge 202 in FIG. 2D makes contact only with a small portion of the top of the right-hand cylinder. Similarly, FIG. 2E is a side view of the filament assembly of FIG. 2A when one of the cylinders is about 1.5% longer than the other. Once again, the flat bridge 202 contacts only a portion of the top of the longer cylinder. In addition, the flat bridge 202 contacts only a portion of the top of the shorter cylinder.
Under such non-ideal conditions, caused by very small imperfections, the resistance of the bridge-to-cylinder connections is greatly increased. If the power supply is configured to provide only enough voltage to heat the filament under ideal conditions, then when a non-ideal condition such as FIG. 2D or FIG. 2E arises the power supply may not be able to provide enough voltage to initiate the filament heating process.
Furthermore, under such non-ideal conditions, the distribution of the current proximal to the bridge-to-cylinder connections is highly uneven, which leads to uneven heating and thermal stresses, as well as uneven deposition of silicon in the connection region, thereby reducing the strength and longevity of the connection. The additional thermal stresses can even result in failure of the bridge and premature termination of the silicon deposition process.
What is needed, therefore, is a tubular filament assembly for a CVD silicon deposition reactor that provides consistent, low resistance connections between the elements of a filament assembly.